The present invention relates generally to an improved molten metal spray forming atomizing ring converter and, more particularly, to such a converter particularly adapted for spray forming of a refined molten metal from a molten metal refining or melting chamber. More particularly, it relates to a spray forming process in which molten metal is atomized into tiny molten droplets by gas impingement on a stream of molten metal and to the means by which the molten metal droplets are preferentially directed to and deposited on a target surface. Most particularly, it relates to controlling the flow of liquid metal droplets and avoiding a backflow of such droplets during the gas atomization.
A molten metal spray forming converter is employed to convert a molten metal stream into an expanding metal spray or plume of small molten metal droplets which impinge and deposit on an appropriate collector which can provide a large metal billet or other object of desired metal characteristics.
One example of molten metal refining is referred to as electroslag refining, and is illustrated and described in U.S. Pat. No. 5,160,532--Benz et al, assigned to the same assignee as the present invention.
In an electroslag process, a large ingot of a preferred metal may be effectively refined in a molten state to remove important impurities such as oxides and sulfides which may have been present in the ingot. Simply described, electroslag refining comprises a metal ingot positioned over a pool of molten metal in a suitable vessel or furnace where the molten pool which may include a surface layer of solid slag, an adjacent underlayer of molten slag and a lowermost body of refined molten ingot metal. The ingot is connected as an electrode in an electrical circuit including the molten metal pool, a source of electrical power and the ingot. The ingot is brought into contact with the molten slag layer and a heavy electrical current is caused to flow across the ingot/molten slag interface. This arrangement and process causes electrical resistance heating and melting of the ingot at the noted interface with the molten ingot metal passing through the molten slag layer as a refining medium to then become a part of the body of refined ingot metal. It is the combination of the controlled resistance melting and the passage of molten ingot metal through the molten slag layer which refines the ingot metal to remove impurities such as oxides, sulfides, and other undesirable inclusions.
In metal spray forming, a small stream of refined molten metal from the furnace is caused to pass concentrically through a molten metal spray forming converter generally comprising a closed peripheral manifold having an open central portion. The manifold is equipped with gas inlet means and plural gas jet exit means. A gas under pressure is supplied to the manifold to exit through the gas jets in converging streams which impinge the passing metal stream to convert or break up the metal stream into a generally expanding spray pattern of small molten metal droplets. This spray pattern can then be directed to impinge and deposit on a suitable collector surface to generate a metal billet or other metal object.
The art of spray forming of metals is a well-developed art. A number of patents have issued relating to this art including the following U.S. Pat. Nos.: 3,909,921, 3,826,301, 4,926,923, 4,779,802. These and other patents dealing with the subject of spray forming provide part of an extensive technical background in this subject matter.
Some of these patents and, particularly, the last mentioned patent deal with the problem of achieving precise control of the mass deposition of the metal on the deposition surface. As is pointed out in U.S. Pat. No. 4,779,802, one proposal to improve the control of the mass distribution of the deposited layer of gas atomized metal is set out in British patent specification 1455862 where it is proposed to isolate the spray of atomized particles by the use of a primary set of gas jets for atomization and two sets of secondary jets which are rapidly switched on and off to impart an oscillatory motion to the spray of atomized metal. However, as noted in the U.S. patent referenced above, it was found that the arrangement did not give ideal control of the mass distribution of the metal deposited. Therefore, an alternative proposal for imparting a direction to a spray was suggested as disclosed in European patent application 0127303A. That arrangement involved the switching on and off of individual gas jets which accomplish the function of both atomizing and oscillating the spray. However, both these methods are very difficult to control and, in particular, lack flexibility in operation. One problem with the first proposal is that the use of secondary jets can result in excess cooling of the deposited metal meaning that subsequently arriving particles do not coalesce and properly adhere to the already deposited metal. The problem with the second method is that the shape and properties of the spray can change as individual jets are switched on and off and this makes it extremely difficult to ensure uniform deposition and solidification conditions.
The recitation of this and numerous other problems related to spray forming of metal articles and solutions to such problems are set forth in patents recited above and others relating to this field.
It is believed that during the spray forming process that the dynamic pattern of the gas in the spray forming chamber is important for effective atomization of the liquid metal stream. It has been found that entrainment of gases at the high-speed jets causes a recirculating flow resulting in an upward velocity of gas near the center line of the nozzle. This recirculating flow is undesirable because the liquid metal stream may not have the momentum necessary to carry the liquid metal through this region, resulting in backsplash of the metal, meaning that the liquid metal droplets were being propelled upward. This backsplash may cause problems with the nozzle from which the stream of molten metal exits from the furnace because droplets may freeze on its surfaces, blocking the orifice and possibly causing freeze-off. Thus, the nozzle may be blocked or completely frozen off by the backsplash. Similarly, the spray forming connection may be blocked.
Notwithstanding, the presentations of the art discussed above, there continues to be a need for providing entrainment gases for the atomization jets without developing an excessive recirculating flow. Such a system and method should provide for a small gas flow, sufficient to feed the entrainment requirements of the high speed jets so as to reduce or eliminate the flow recirculation near the nozzle that leads to backsplash. At the same time, this flow must be low enough to avoid preatomization of the liquid metal which could result if this gas flow impinges on the liquid metal stream, causing it to break up prior to reaching the atomization zone.